Control rod drives (CRDs) are used to position control rods in boiling water reactors (BWRs) to control the fission rate and fission density, and to provide adequate excess negative reactivity to shutdown the reactor from any normal operating or accident condition at the most reactive time in core life. Referring to FIG. 1, each CRD is mounted vertically in a CRD housing 10 which is welded to a stub tube 8, which in turn is welded to the bottom head of the reactor pressure vessel 4. The CRD flange 6 is bolted and sealed to the flange 10a of the CRD housing 10, which contains ports for attaching the CRD hydraulic system lines 80, 81. Demineralized water supplied by the CRD hydraulic system serves as the hydraulic fluid for CRD operation.
As shown schematically in FIG. 1, the CRD is a double-acting, mechanically latched hydraulic cylinder. The CRD is capable of inserting or withdrawing a control rod (not shown) at a slow controlled rate for normal reactor operation and of providing rapid control rod insertion (scram) in the event of an emergency requiring rapid shutdown of the reactor. A locking mechanism in 12 the CRD permits the control rod to be positioned at 6-inch (152.4-mm) increments of stroke and to be held in these latched positions until the CRD is actuated for movement to a new position. A spud 46 at the top of the index tube 26 (the moving element) engages and locks into a socket at the bottom of the control rod. Once coupled, the CRD and control rod form an integral unit which must be manually uncoupled before a CRD or control rod may be removed from the reactor.
When installed in the reactor, the CRD is wholly contained in housing 10. The CRD flange 6 contains an insert port 66, a withdraw port 70 and an integral two-way check valve (with a ball 20). For normal drive operation, drive water is supplied via an associated hydraulic control unit (HCU) to the insert port 66 for drive insertion and/or to withdraw port 70 for drive withdrawal. For rapid shutdown, reactor pressure is admitted to the two-way check valve from the annular space between the CRD and a thermal sleeve (not shown) through passages in the CRD flange, called scram vessel ports. The check valve directs reactor pressure or external hydraulic pressure to the underside of drive piston 24.
Referring to FIG. 2, the CRD further comprises an inner cylinder 57 and an outer tube 56, which form an annulus through which water is applied to a collet piston 29b (see FIG. 1) to unlock index tube 26. The internal diameter of inner cylinder 57 is honed to provide the surface required for expanding seals 65 on the drive piston 24.
Welded pipes 80 and 81, installed in the CRD housing, port water to the insert port 66 and the withdraw port 70 respectively. A port 69 below outer tube 56 connects to withdraw port 70 in CRD flange 6 so that water is applied through the annulus to collet piston 29b when a withdraw signal is given.
The CRD is secured to the CRD housing flange 10a by eight mounting bolts (not shown). A pressure-tight seal is effected between the mated flanges by O-ring gaskets (not shown) mounted in a spacer 7 secured to the CRD flange face.
Insert port 66 contains a ball check valve which consists of check-valve ball 20, ball retainer 21, and retainer O-ring 22. This valve directs HCU accumulator pressure or reactor pressure to the underside of drive piston 24 during scram operation. Port 66 is connected internally to the annulus and the bottom of drive piston 24 and serves as the inlet for water during normal insertion or scram. Water enters this port for a brief period in response to a withdraw signal to move the index tube 26 upward so that collet fingers 29a (see FIG. 1) are cammed out. Following this brief unlocking period, water from below drive piston 24 is discharged through port 66 and through the under-piston hydraulic line for the duration of the withdraw signal.
The withdraw port 70 serves as the inlet port for water during control rod withdrawal and as the outlet port for water during normal or scram insertion. It connects with internal porting and annuli to the area above drive piston 24. During a withdraw operation, water is supplied from port 70 through a small connecting port in CRD flange 6 to the annular space between outer tube 56 and inner cylinder 57 for application to the bottom of collet piston 29b.
The locking mechanism comprises collet fingers 29a, collet piston 29b and collet spring 31. This mechanism is the means by which index tube 26 is locked to hold the control rod at a selected position.
The collet mechanism requires a hydraulic pressure greater than reactor pressure to unlock for CRD-withdraw movement. A preload is placed on collet spring 31 at assembly and must be overcome before the collet can be moved toward the unlocked position. For control rod withdrawal, a brief insert signal is applied to move index tube 26 upward to relieve the axial load on collet fingers 29a, camming them outward against the sloping lower surface of index tube locking notch 55. Immediately thereafter, withdraw pressure is applied. In addition to moving index tube 26 downward, this pressure is at the same time applied to the bottom of collet piston 29b to overcome the spring pressure and cam the fingers 29a outward against a guide cap (not shown). When the withdraw signal ceases, the spring pressure forces the collet downward so that fingers 29a slip off the guide cap. As index tube 26 settles downward, collet fingers 29a snap into the next higher notch and lock. When collet fingers 29a engage a locking notch 55, collet piston 29b transfers the control rod weight from index tube 26 to the outer tube 56.
Unlocking is not required for CRD insertion. The collet fingers are cammed out of the locking notch as index tube 26 moves upward. The fingers 29a grip the outside wall of index tube 26 and snap into the next lower locking notch for single-notch insertion to hold index tube 26 in position. For scram insertion, index tube 26 moves continuously to its limit of travel during which the fingers snap into and cam out of each locking notch as index tube 26 moves upward. When the insert, withdraw or scram pressures are removed, index tube 26 settles back, from the limit of travel, and locks to hold the control rod in the required position.
The drive piston 24 and index tube 26 are the primary subassembly in the CRD, providing the driving link with the control rod as well as the notches for the locking mechanism collet fingers. Drive piston 24 operates between positive end stops, with a hydraulic cushion provided at the upper end only. Index tube 26 is a nitrided stainless-steel tube threaded internally at both ends. The spud 46 is threaded to its upper end, while the head of the drive piston 24 is threaded to its lower end. Both connections are secured in place by means of bands 25 with tab locks.
There are 25 notches machined into the wall of index tube 26, all but one of which are locking notches 55 spaced at 6-inch intervals. The uppermost surfaces of these notches engage collet fingers 29a, providing 24 increments at which a control rod may be positioned and preventing inadvertent withdrawal of the rod from the core. The lower surfaces of the locking notches slope gradually so that the collet fingers cam outward for control rod insertion.
Drive piston 24 is provided with internal (62, 71, 72) and external seal rings (65), and is operated in the annular space between piston tube 15 and inner cylinder 57. Internal (63) and external (64) bushings prevent metal-to-metal contact between drive piston 24 and the surface of piston tube 15 and the wall of inner cylinder 57 respectively.
The magnet housing, which comprises the lower end of drive piston 24, contains a ring magnet 67 which actuates the switches of the position indicator probe (not shown) to provide remote electrical signals indicating control rod position.
The piston tube assembly forms the innermost cylindrical wall of the CRD. It is a welded unit consisting of piston tube 15 and a position indicator tube 61. The position indicator tube 61 is a pressure-containing part which forms a drywell housing for a position indicator probe 12a (see FIG. 2). Piston tube 15 provides for the porting of water to or from the upper end of the piston head portion of drive piston 24 during rod movement.
The tube section 15a and head section 15b of piston tube 15 provide space for position indicator tube 61, which is welded to the inner diameter of the threaded end of head section 15b and extends upward through the length of tube section 15a, terminating in a watertight cap near the upper end of the tube section. Piston tube 15 is secured by a nut 16 at the lower end of the CRD. Two horizontal ports are provided in the head section 15b, 180.degree. apart, to transmit water between the withdraw porting in the CRD flange and the annulus between indicator tube 61 and tube section 15a of piston tube 15 for application to the top of drive piston 24. Three O-ring seals 18 are installed around head section 15b. Two seal the bottom of the CRD against water leakage and one seals the drive piston 24 under-piston pressure from the drive piston over-piston pressure.
The position indicator probe 12a, which is slidably inserted into indicator tube 61, transmits electrical signals to provide remote indications of control rod position and CRD operating temperature. Probe 12a is welded to a plate 12b, which is in turn bolted to housing 12. Housing 12 is secured to the CRD ring flange 17 by screws 13. A cable clamp (not shown), located at the bottom of a receptacle 14, secures a connecting electrical cable to receptacle 14. Ring flange 17 is in turn secured to the CRD housing by screws 9. Thus, probe 12a, housing 12 and the cable clamp (with the cables passing therethrough) can be removed as a unit.
In order to perform maintenance on a CRD, the CRD must be removed from its CRD housing. After the CRD and control rod are uncoupled, the CRD is removed from the CRD housing and then disassembled for the purpose of performing required maintenance on the respective CRD components. The CRD is placed in a horizontal position on a work table and disassembled. After drive piston 24 has been disengaged from the index tube and removed from the piston tube, it is disassembled into its component parts, i.e., magnet housing 24a, seal cups 24b, piston coupling 24c and piston head 24d (see FIG. 3).
After piston coupling 24c has been disengaged from magnet housing 24a and piston head 24d, the internal bushings 63 must be removed from the piston coupling. These bushings are graphitar rings which sit in annular recesses located at opposing ends of the piston coupling bore. These rings become encrusted with crud during reactor operation. As a result, the internal bushings become stuck inside the piston coupling and are difficult to remove using conventional tools such as chisels and screwdrivers. Moreover, during such removal, the piston coupling may be damaged.